The present invention relates to a process for sterilization of medical instruments using a chemical sterilant. More particularly, the invention relates to a process in which sterilization is achieved by concentrating a sterilant such as hydrogen peroxide inside of a sterilization chamber and sterilizing articles therewith.
Medical instruments have traditionally been sterilized using either heat, such as is provided by steam, or a chemical, such as formaldehyde or ethylene oxide in the gas or vapor state. Each of these methods has its drawbacks. Many medical devices such as fiberoptic devices, endoscopes, power tools, etc., are sensitive to heat, moisture or both. Formaldehyde and ethylene oxide are both toxic gases that pose a potential hazard to healthcare workers. Problems with ethylene oxide are particularly severe, because its use requires long aeration times to remove the gas from articles that have been sterilized. This lengthens the sterilization cycle time undesirably.
Sterilization using liquid hydrogen peroxide solution has been found to require high concentrations of sterilant, extended exposure time and/or elevated temperatures. However, sterilization using hydrogen peroxide vapor has been shown to have some advantages over other chemical sterilization processes (see, e.g., U.S. Pat. Nos. 4,169,123 and 4,169,124, each of which issued Sep. 25, 1979, which are entitled respectively, xe2x80x9cHydrogen Peroxide Vapor Sterilization Methodxe2x80x9d and xe2x80x9cCold Gas Sterilization Processxe2x80x9d and which are incorporated herein by reference).
The combination of hydrogen peroxide with a plasma provides certain additional advantages, as disclosed in U.S. Pat. No. 4,643,876 issued Feb. 17, 1987 and entitled, xe2x80x9cHydrogen Peroxide Plasma Sterilization Systemxe2x80x9d which is incorporated herein by reference. Commercially available sterilization devices, such as the STERRAD(copyright) sterilization systems sold by Advanced Sterilization Systems division of Ethicon, Inc. automate the process of injecting a solution of hydrogen peroxide into a sterilization chamber, vaporizing the solution to provide a hydrogen peroxide vapor, contacting articles to be sterilized with the vapor, and exciting the vapor into the plasma stage. The hydrogen peroxide for each sterilization cycle is shipped to the location of the sterilization system, generally by air or ground transportation.
Preferably, as in the case with the STERRAD(copyright) brand systems, pre-measured amounts of a hydrogen peroxide and water solution are provided in sealed enclosure, such as a capsule inside of a cassette housing which can be automatically opened by the system to reduce contact between the system user and the hydrogen peroxide solution. Such cassettes are described more fully in U.S. Pat. No. 4,817,800 issued Apr. 4, 1989 entitled, xe2x80x9cFluid Injection System Cassette and Fluid Packaging Methodsxe2x80x9d and U.S. Pat. No. 4,899,519 issued Feb. 13, 1990 with the same title, each of which are incorporated herein by reference.
The sterilization of articles containing diffusion-restricted areas, such as long narrow lumens, presents a special challenge. Methods that use hydrogen peroxide vapor that has been generated from an aqueous solution of hydrogen peroxide have certain disadvantages. One disadvantage is that because water has a higher vapor pressure than hydrogen peroxide, it will vaporize faster. Another disadvantage is that because of its lower molecular weight, water will diffuse faster than hydrogen peroxide in the vapor state. Because of these physical properties, when an aqueous solution of hydrogen peroxide is vaporized in the area surrounding the items to be sterilized, the water reaches the items first and in higher concentration The water vapor more quickly diffuses into and thus inhibits penetration of hydrogen peroxide vapor into diffusion-restricted areas, such as small crevices and long narrow lumens. Simply employing a more concentrated solution of hydrogen peroxide fails to adequately address the problem due to the difficulty in handling highly concentrated hydrogen peroxide solutions. Transportation of such solutions can be particularly difficult. In general, such solutions are limited to concentrations of less than 60% hydrogen peroxide, however, regulations and the like regarding such concentrations may of course be modified in the future. In any event, shipping and handling of highly concentrated solutions remains impractical.
U.S. Pat. No. 4,952,370 issued Aug. 28, 1990 and entitled xe2x80x9cHydrogen Peroxide Sterilization Methodxe2x80x9d and incorporated herein by reference discloses a sterilization process in which aqueous hydrogen peroxide vapor is first condensed on the article to be sterilized, followed by application of a vacuum to the sterilization chamber to remove the water and hydrogen peroxide from the article. This method is suitable for surface sterilization, but not for sterilization of diffusion-restricted areas such as long narrow lumens because it depends on the diffusion of hydrogen peroxide vapor into the lumen to effect sterilization.
U.S. Pat. No. 4,943,414 issued Jul. 24, 1990 and entitled xe2x80x9cMethod for Vapor Sterilization of Articles Having Lumensxe2x80x9d discloses a process in which a vessel containing a small amount of a vaporizable liquid sterilant solution is attached to a lumen, and the sterilant vaporizes and flows directly into the lumen of the article as the pressure is reduced during the sterilization cycle. This system has the advantage that the water and hydrogen peroxide vapor are pulled through the lumen by the existing pressure differential, increasing the sterilization rate for lumens, but has the disadvantage that the vessel needs to be attached to each lumen to be sterilized.
U.S. Pat. No. 5,492,672 issued Feb. 20, 1996 and entitled, xe2x80x9cSterilization Apparatus and Method for Multicomponent Sterilantxe2x80x9d discloses a process for sterilizing narrow lumens. This process uses a multi-component sterilant vapor and requires successive alternating periods of flow of sterilant vapor and discontinuance of such flow. A complex apparatus is used to accomplish the method. Because flow through of vapor is used, closed end lumens are not readily sterilized in the process.
U.S. Pat. No. 4,744,951 issued May 17, 1988 to Cummings and entitled xe2x80x9cVaporization method to Enhance Sterilant Penetrationxe2x80x9d attempts to address this problem by providing a separate prechamber connected to the sterilization chamber. Hydrogen peroxide is first admitted to the prechamber where it is concentrated in a distillation procedure employing the differing vapor pressures of hydrogen peroxide and water. Water""s higher vapor pressure allows one to select a vaporization pressure that selectively vaporizes water from a hydrogen peroxide solution, thus concentrating the solution. Cummings pumps air out of the prechamber and lowers its pressure to a level at which the water preferentially vaporizes from the hydrogen peroxide solution. The pump that is evacuating the prechamber draws out the water vapor thus released from solution to concentrate the remaining solution. To prevent the water vapor from traveling into the narrow spaces such as endoscope lumens, Cummings carries out the concentration process in the prechamber which is physically isolated from the main chamber. This adds complexity by requiring additional chambers, pumps and valves.
Those of skill in the art would not think to employ such a concentration process in the same chamber as the sterilization occurs. Such a process first draws the water out of solution and it would have been thought that this water vapor would simply enter and thus occlude the narrow lumens, thereby inhibiting the later diffusion of hydrogen peroxide, no matter how concentrated, into those lumens. However, the present inventors have surprisingly found that concentrating the hydrogen peroxide vapor within the sterilization chamber greatly increases the ability to sterilize long narrow lumens over the conventional process.
One aspect of the present invention comprises a method of furnishing concentrated hydrogen peroxide vapor to an article. It includes the following steps:
placing the article into a chamber containing an inner atmosphere;
placing a solution comprising hydrogen peroxide and water into fluid communication with the chamber, the solution having a ratio of hydrogen peroxide to water;
vaporizing the solution in the inner atmosphere to form water vapor and hydrogen peroxide vapor;
selectively drawing water vapor from the chamber to increase a ratio of hydrogen peroxide to water in the chamber; and
contacting the article with the hydrogen peroxide vapor.
Preferably, the ratio of hydrogen peroxide vapor to water vapor after the step of selectively drawing water vapor from the chamber exceeds the ratio of hydrogen peroxide to water in the solution. Preferably, the ratio of hydrogen peroxide to water, by weight, after the step of selectively drawing water vapor from the chamber exceeds 3 to 1, more preferably 4 to 1; and the ratio of hydrogen peroxide to water in the solution, by weight, is preferably less than 3 to 1, more preferably less than 3 to 2.
The step of selectively drawing water vapor from the chamber, preferably comprises placing the solution within a diffusion restricted environment in fluid communication with the chamber during the step of vaporizing the solution; the diffusion restricted environment is preferably more diffusion restricted during the step of selectively drawing water vapor from the chamber than during a portion of the step of vaporizing the solution without selectively drawing water vapor from the chamber.
In one aspect of the invention the water vapor is drawn from the chamber through one or more exhaust ports and wherein the one or more exhaust ports are physically remote from the diffusion restriction.
Preferably, the temperature and pressure of the solution are controlled during the step of vaporizing the solution to enhance vaporization of the water from solution versus vaporization of hydrogen peroxide to thus extract at least a portion of the water vapor from the chamber.
One aspect of the invention comprises maintaining the solution at a pressure below the vapor pressure of the water in the solution and above the vapor pressure of the hydrogen peroxide in the solution while selectively drawing water vapor from the chamber. To increase the vapor pressure of the water in the solution relative to the hydrogen peroxide in the solution and thus enhancing the ability to selectively vaporize and extract the water vapor, the temperature of the solution can be held below the temperature of the atmosphere in the chamber. For instance, the temperature of the atmosphere in the chamber can be above room temperature and the temperature of the solution during the vaporizing step can be held at least 10xc2x0 C. below the temperature of the atmosphere in the chamber. One way to achieve this is to thermally isolate the vaporizer from the chamber, y isolated from the chamber.
In one aspect of the invention, the temperature and pressure of the solution are controlled during a least a first portion of the vaporizing step so as to selectively vaporize water from the solution and concentrate hydrogen Deroxide therein to form a concentrated solution and during a second portion of the vaporizing step the temperature of the concentrated solution is raised and then vaporized. Preferably, after the solution is concentrated, no more atmosphere is evacuated from the chamber so as to conserve hydrogen peroxide.
Preferably, the chamber is dried prior to vaporizing the solution, such as by pumping a portion of the atmosphere out of the chamber or by applying energy to excite molecules of liquid water within the chamber into the gaseous or plasma state of matter and pumping a portion of the atmosphere out of the chamber.
In one aspect of the invention, the step of contacting the article with the hydrogen peroxide vapor is limited to less than one hour and if the article were to have a straight round lumen having two open ends, a diameter of 1 mm and a length of 250 mm with 106 viable spores of B. Stearothermophilus located within the lumen at a midpoint thereof, all of the spores would be killed.
In one aspect of the invention the solution comprises peracetic acid.